This invention relates to rolling bearings, more particularly to rolling bearings used for rotary shafts of information devices such as hard disk drive devices (HDD), video tape recorders (VTR), digital audio tape recorders (DAT), and laser beam printers (LBP), and high-speed rolling bearings for supporting small-diameter rotary shafts rotating at a high speed and used with magnetic recording/reproducing devices, and greases for such rolling bearings.
Usually, for lubrication of rolling bearings used in information devices, lubricating oil and/or semisolid lubricating oil such as grease is used. With reducing size and improving performance of information devices, acoustic levels required for bearings for rotary shafts are becoming severer year by year.
For example, it is described in unexamined Japanese patent publication 8-283767 that bearings exhibit excellent acoustic properties during high-temperature, high-speed rotation by using a lubricant composition containing as an essential component an ester oil having a dynamic viscosity at 40xc2x0 C. of 10 mm2/s or over.
FIG. 1 shows a structure for supporting rotary shafts of a hard disk drive device (hereinafter abbreviated to xe2x80x9cHDDxe2x80x9d). This support structure has two rotary shafts, i.e. a rotary shaft 3 rotatably supporting a magnetic disk 2 through a hub 1, and a rotary shaft 5 for an arm 4 carrying a magnetic head (not shown) on its tip. These rotary shafts 3, 5 are supported by a pair of ball bearings 6, 7, respectively, each pair of bearings being arranged axially spaced from each other.
The inner rings 8 of the pair of ball bearings 6 supporting the rotary shaft 3 are mounted so as to rotate together with the rotary shaft 3, and the outer rings 9 are fixedly fitted in a tubular fixing element 11 of a spindle motor 10, and the rotary shaft 3 is fixed to the center of a deep dish-shaped rotary element 12 to rotate the rotary shaft 3 by the spindle motor 10.
Rotatably supported by such a support structure, the magnetic disk 2 rotates at high speed corresponding to the rotating speed of the spindle motor 10. The arm 4 (called swing arm) carrying a magnetic head for reading and writing magnetic data works suitably with the magnetic disk 2.
The end of the arm 4 is supported by the top of the rotary shaft 5. The latter is rotated by an actuator (not shown) comprising a coil about its axis to swing the tip of the arm 4 a predetermined angle, thereby moving the magnetic head to a required position. By this rotation of the rotary shaft 5, the magnetic head can read and write magnetic data in an effective record area of the magnetic disk 2.
For conventional rolling bearings for supporting rotary shafts of HDDs, high-precision miniature ball bearings (see JIS B 0104 xe2x80x9cball bearings having an outer diameter less than 9 mmxe2x80x9d) are used. At least for dimensional and rotating accuracy, those in the JIS class 5 or higher are required.
It is described in unexamined Japanese patent publication 8-283767 that the bearing can suppress dust, reduce torque and extend the life while maintaining good acoustic properties even during high-temperature, high-speed rotation. But, since acoustic deterioration during high-speed rotation was not considered sufficiently, selection of lubricating oil was a problem. Normally, when a bearing is rotated at high speed for some period of time, it becomes difficult to form a good oil film. Thus the acoustic level after operation for a certain time period worsens compared with the initial acoustic level. Thus, if the kind of lubricating oil and its dynamic viscosity are not properly selected, the acoustic level may considerably deteriorate after operation at high-speed rotation for a certain time.
An object of the present invention is to provide a rolling bearing in which the acoustic deterioration is suppressed while maintaining low dust, low torque and long life.
For bearings used in devices for recording and reproducing electronic information such as HDDS, VTRS, LBPs, CDs and DVDS, with requirements for compactness of these devices, low torque is also desired to save power consumption.
Also, in such rolling bearings, lubricating oil or grease is used by applying or filling in as small an amount as possible to reduce the rotation torque of the rolling bearing. Even if only a small amount of lubricant scatters by centrifugal force from the bearing of an electronic information recording/reproducing device which is rotating at high speed, lubrication tends to become insufficient.
If lubricating oil is short on the friction surface of the small-diameter rolling bearing which is rotating at high speed, the rotation torque of the bearing will increase rapidly, thus affecting badly on the precision mechanisms of the electronic information recording/reproducing devices. This increases the possibility of malfunction and deteriorates reliability in recording/reproducing of electronic information.
Further, if lubricating oil becomes scarce on the friction surface of the rolling bearing, noise will be produced at the same time.
Another object of the present invention is to provide a grease for rolling bearings which is less likely to scatter under centrifugal force at high speed rotation, whereby preventing malfunction and noise of electronic information devices and which extends the life of this type of rolling bearings.
According to this invention, a lubricating oil film of an ester-series oil having a dynamic viscosity at 40xc2x0 C. of 35 mm2/s or over is formed on the friction surface of a rolling bearing. The bearing may be a ball bearing having an inner ring having an inner diameter of 8 mm or under. Further, grease may be sealed in the bearing.
Such a rolling bearing using such an ester-series oil as lubricating oil can suppress acoustic deterioration during high-speed rotation.
According to the present invention, there is also provided a high-speed rolling bearing in which a lubricating grease having its worked penetration (JIS K2220) adjusted to 175-250 is retained therein.
According to the present invention, a grease for a high speed rolling bearing having its worked penetration (JIS K2220) adjusted to 175-250 is also provided.
The grease is preferably prepared by adding a thickening agent comprising a lithium soap or an urea to a base oil comprising an ester oil having a dynamic viscosity at 40xc2x0 C. of 10 mm2/s or over.
The grease is more preferably prepared by adding a thickening agent containing 50 wt % or more of an aliphatic lithium salt 12-24 in carbon number to a base oil comprising 30 wt % or more of an ester oil having a dynamic viscosity at 40xc2x0 C. of 10 mm2/s or over, and having its worked penetration (JIS K2220) adjusted to 175-250.
The grease of this invention, the worked penetration of which has been adjusted to a predetermined range, maintains a predetermined consistency when agitated in a rolling bearing rotating at high speed and subjected to shearing. Thus it is less likely to scatter or leak out of the bearing. This prevents insufficient lubrication of the bearing.
With the grease, in which a base oil preferably containing 30 wt % or more of an ester oil having a dynamic viscosity at 40xc2x0 C. of 10 mm2/s or over is used, the lubricating oil (base oil) supplied onto the friction surface from the grease has a predetermined dynamic viscosity, so that a lubricating oil film of a predetermined thickness is formed on the friction surface. This improves lubricity and reduces and stabilizes turning torque while the rolling bearing is rotating at high speed.
Further, the grease to which is added a thickening agent containing 50 wt % or more of an aliphatic lithium salt 12-24 in carbon number, due to excellent thickening properties of the thickening agent, shows stable consistency at high temperatures.
Due to the fact that the lubricating grease is less likely to scatter and leak, the high-speed rolling bearing according to this invention can rotate at high speed with low torque without causing poor lubrication and producing noise. The rolling bearing enjoys a long life.
Further, it prevents malfunction of electronic information recording/reproducing devices during recordation and reproduction.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawing, in which: